Influenza A in Swedish harbour seals (Phoca vitulina) : a serological study

Sammanfattning: Evidence of infection with influenza A has been documented in several different marine mammals. However, large outbreaks with mass mortalities have only been seen in harbour seals. In 2014, the first outbreak with influenza A virus (IAV) in harbour seals in Europe was reported. The outbreak started in coastal waters off the Swedish west coast and spread to Denmark, Germany, and the Netherlands. More than 2 000 harbour seals died during the event. The aim of this study was to do a follow-up on the outbreak with IAV in Swedish harbour seals in 2014. The overall purpose was to increase the knowledge about IAV infections in the Swedish harbour seal populations. More specific objectives included mapping circulation of specific subtypes and evaluating cadaver blood as a sample material instead of pure serum for IAV antibody detection and subtyping. Serum samples were acquired from live-captured seals in 2014, and cadaver blood samples from hunted or found dead seals from 2014-2021. All samples were screened for anti-nucleoprotein antibodies using a commercial competitive ELISA. Positive and doubtful samples were confirmed and subtyped by hemagglutination inhibition assay (HI). In addition, information from sampled seals was gathered to draw relevant conclusions from test results. The results show a high level of antibodies against IAV H10N7 in a large proportion of serum samples from 2014, with declining levels of antibodies against IAV in a lower proportion of cadaver blood samples from the west coast in 2015 and 2016. IAV antibodies were not found in samples from the Kalmarsund population or in the population off the west coast after 2016. Half of the ELISA positive or doubtful cadaver blood samples could be confirmed and subtyped by HI. Data from sampled seals made it possible to connect all ELISA positive samples to exposure to the outbreak in 2014. Evidence of circulation of other IAV subtypes was not found. This study indicates that IAVs do not circulate and persist within the Swedish harbour seal populations, and that the populations most likely are naive in the case of a possible new spillover event. Environmental contaminants, low genetic variation, larger populations, and smaller habitats are identified as important risk factors for increased incidence of disease and spread of infection in a potential new outbreak. Cadaver blood is an accessible sample which according to this study can be useful in IAV surveillance on a population level to determine whether a population has been exposed to the IAV or not. It can therefore be an important tool for understanding disease dynamics, disease control and possibly prevention of pandemic influenza.